The subject matter disclosed herein relates generally to industrial control networks for controlling machines and industrial processes and, more specifically, to using time synchronization to allocate and prioritize data communications on the industrial control network for a distributed control system.
Industrial controllers are special purpose computers used for controlling factory automation and the like. Industrial controllers typically execute a control program highly customized to a particular control application. Special control languages, such as “relay ladder logic” are normally used to facilitate programming of the device. Under the direction of the stored program, a processor of the industrial controller periodically examines the state of input devices and updates the state of output devices. In order to ensure predictable control of a machine or process, the control program must be highly reliable and execute at well-defined time periods.
As industrial processes grow in complexity, an increasing number of devices are being connected to the industrial controller. The devices are often distributed about a machine or along a process line. An industrial network is typically used to connect the distributed devices and to allow data to be transmitted between the devices. In order to maintain reliability of the control program, communications between distributed devices must similarly be highly reliable and execute at well-defined time periods.
In response to the growing complexity of industrial networks, numerous industrial communication protocols have been developed to provide highly scheduled communications between distributed devices. These industrial protocols reserve time slots for individual devices to communicate on the network, ensuring that all critical communications can occur during a predefined time.
While these industrial communication protocols satisfy the structured demands of industrial communications, they are not without their drawbacks. Highly scheduled communications requirements restrict flexibility of industrial control systems. As the size and complexity of systems grow, an ever increasing number of communication and scheduling requirements, based on the interactions between devices, requires complex, recursive scheduling optimization routines. If future expansion of the system is desired, the entire schedule must be reconfigured to accommodate the new devices. Further, the expansion may have topology restrictions to ensure that the available bandwidth of the network meet performance requirements along each branch of the network. Thus, it is desirable to provide a system that satisfies the deterministic requirements of an industrial control system and retain flexibility for ease of setup and expansion.